Double click the drop down list box and select the data source connection you created for the SharePoint list to retreive data from
Click the RULES button to create a rule for this drop down list box
Add rule - add condition - action (select the field's value as) - field (select the field you want have the display. In this case its item description text box)
Click on Fx button to insert formula - click insert field group - select data source (secondary) - select description field - click on filter data button - click add - select ID from secondary data source is equals to Description from the secondary data source
Click on OK button untill you return to following screen
Click on EditXPath (advanced) check box and change the code as follows
xdXdocument:GetDOM("Inventory Item")/dfs:myFields/dfs:dataFields/dfs:Iventory_Item/@Description[../@ID = current()]
Finally when you see the preview of created InfoPath form, you should be able to retrieve Item description by selecting Item code in drop down list box.